I recently got into a discussion again about the question of monogenism and how modern science affects (or doesn't affect) the notion that original sin is transmitted by descent. I have discussed before how there is really no problem, because saying that humanity comes from one couple does not necessarily imply a genetic bottleneck. However, that's just what I was thinking about recently: the Church's teaching on original sin and monogenism, especially in the context of what we now know about evolution, is really ultimately a teaching on just what it means to be human (as opposed to merely hominid) and how humanity is transmitted.
Monogenism, then, is less a historical biological or genetic claim, and more a definition of just who constitutes a human, a claim on what the meaning or parameters of "humanity" are. In this sense, it is (like other doctrines) scientifically unfalsifiable, because it is a sort of "by definition" claim: humanity is defined to be transmitted by descent from one couple, therefore any and only descendents from that couple are by definition human. In itself, this is unfalsifiable.
Of course, the "problem" that arises is that we certainly don't want to claim that anyone alive today (or at any point in recorded history) is not a human, is some sort of unensouled "philosophical zombie" hominid. And, indeed, it would seem to disprove the idea that humanity is transmitted only by descent if there were people walking around fully recognized as human, and integrated into the human web of meaning (through use of language, etc), but who were "really not" human.
Because "humanity" is ultimately a construct. This should be obvious enough to us living today as moderns (and has relevance to the abortion debate, of course), and it is the implication of what I was saying about "by definition" monogenism above. The teaching is really more about the meaning of the construct, not about a historical claim. The historical claim only enters into the teaching accidentally, as it were, because of the definition of "humanity" as something transmitted only by descent, a community of meaning one can only enter into through birth, at least after the founding pair whose existence and ensoulment I now tend to think of in terms of something like Hegel's belief (according to Wikipedia's summary, lol) that "spirit cannot come to be without first a self-consciousness recognizing another self-consciousness."
I'd argue that this is an ontological and metaphysical leap that (besides being attributable only to God directly), also could only occur once. At least, in terms of any Meaning that would be mutually recognizable or intelligible to us. I imagine Adam and Eve in their tribe of hominids making this leap in recognizing each other (becaus, we must say of course, they had been ensouled by God from the start). But if we were to imagine, say, another pair or pairs of different hominids making this same "leap" in other mutual recognitions...each of these encounters would actually constitute a radically different "root" from which a totally unique meaning (or, rather, its equivalent, if we are defining "meaning" as uniquely human), a unique "school of intersubjectivity" would result. They would not at all result in the same meaning, the same intersubjectivity, being transmitted, exactly because of the diversity of the subjects (or their equivalents) in question. This is similar to what I've said about extraterrestrial "intelligences" likely being entirely meaningless to us as humans or in human terms.
"Humanity" and human subjecthood in the human community of meaning...is actually, in this sense, "relative" or relational, and not some sort of absolute or objective essence of an evolutionary definitional line crossed. We're human through membership in the community of humanity. Humanity can only mean entering into an intersubjectivity that could (by the very fact of being an intersubjectivity) only possibly have been "founded" by a single relationship, and which could thus only be entered into through being born "into the bosom of" that intersubjectivity already.
After the initial founding of the relationship it, by nature, is not something that could be "entered" from the outside, nor could it be "conferred" by the original pair on others who had not been part of the original intersubjective encounter between the two (as all that would create, at most, is new "root" intersubjectivities, which would create irreducibly unique "lines" of meaning or, rather, meaning-equivalent). New membership in this intersubjectivity could thus only come about by being procreated "into" (and resulting from) the encounter itself. By one parent, at least; other animal-hominid lines could have bred in genetically. However, this is one reason to assume intersubjectivities of any sort, "parallel" communities of subjecthood(-equivalent), were never achieved by any other pairs: if it had been, cross-breeding between the two lines would have resulted in an offspring who was a subject in both, who would thus have had two "natures" (just as the One with a human mother and divine Father had both natures.)
This, I think, is the understanding that the doctrine on monogenism upholds so necessarily. It refutes the error that humanity is something that could be entered "from the outside" or which the human community has the power to confer on those it chooses to initiate or recognize. It does not. And while this means we can never by fiat make gorillas or dolphins human...the really important implication is that we can never take humanity away from any organism which descends from humans. There are no monsters, there can never be a soulless non-human born of man. If there is a living body descended from humanity, it is human, even if it is a "brain-dead" patient or an anencephalic infant without a brain.
(Some questions are raised about all this by the advent of biotechnology such as cloning. Obviously, a clone is still created from human tissue so that's not really an issue anymore than the fact that identical twins result from a "splitting" rather than direct conception. More troubling, though, is the idea of "building" a human out of non-human-derived organic chemicals "from scratch" in a laboratory. Personally, I would assume that this could still be called human given that it was created according to the human template by human agency...but I hope we never go so far in playing God!)
This, I think, is really the content of the teaching on monogenism (and, thus, original sin, which merely says that the human intersubjectivity has been marred from nearly the start by sin, which causes "humanity" to be mortal, originally without grace, etc.) The necessary historical claim going along with this that "all humans properly so-called descend from an original pair in at least one line of descent, have at least that pair as common ancestors," really only would be problematic from a scientific perspective if humanity as recognized throughout history could not be shown to have any common ancestors, or at least any common ancestors who themselves could conceivably be called human (ie, at least hominid; obviously if you go back far enough, any two organisms have common ancestors, but if we found out that the most recent common ancestor of all humans was a dog, and/or that the most recent common ancestry to include all humans also included today's chimps...this would be problematic).
However, this claim is not at all a scientific problem for all humans alive today (whose Most Recent Common Ancestor may even have been after Christ). Assuming we also need to include all humans alive during the period of recorded history pushes the question of a Most Recent Common Ancestor back further, and indeed we probably want to say that humanity was already universal before the original population spread from its home territory (lest there be any idea of independent non-human "lines" potentially subsisting anywhere). But in actuality there is no current scientific dispute that the original population from which all later humans descended was a small population of only a few thousand in Africa about 300,000 years ago, and almost certainly in a population so small it would have taken relatively few generations for there to be a pair (itself from an earlier point in the population) of most recent common ancestors for the whole population. Indeed, before spreading out from Africa the population likely had many ancestors common to all of them from earlier in the population's history, and indeed an Identical Ancestors Point had probably been established that itself was from within the same population.
So the science really isn't a problem here, and disputing about it is a distraction from the important part of the teaching to which the historical claim is merely a sort of necessary corollary. And yet people are quick, even eager I'd say, to want to believe that science has "disproven" the Church's dogma here, as if the lack of a mere genetic two-person bottleneck means we need to rethink what it means to be human or how human subjecthood is transmitted. That is simply not true, at least not with what we know about common ancestors (which has always been intuitively apparent; even before we knew about evolution, people always correctly assumed that species of animal and plant had descended from common root ancestors who were at least similar to them in form). Why then this jump to say that the lack of a two-person genetic bottleneck somehow changes the meaning of humanity or mode of transmission of human subjecthood?
Part of it may just be the general incredulity induced by the need to modify the "traditional"/simplistic understanding (which indeed seemed to imagine there was a two person genetic bottleneck). "Well, if we have to adapt things with a complicated biological argument involving unensouled hominids, that's clearly just an attempt to save the appearances and so we might as well question the whole edifice! Everything is up for grabs." This is an unfortunate result of change, even when the change is totally accidental (after all, the essence of the teaching was never about genetic bottlenecks in themselves, as if that's spiritually important, but rather about the nature and origins and definition and transmission of human subjecthood) and why the Church is right to be cautious and very conservative about any sort of evolution of understanding like this (because even changing the accidents of teachings can make people think the dogmatic essences are thus also up for grabs; just look at the scandal caused by "changes" regarding usury, slavery, religious liberty, etc etc).
But I also get a sense that part of it is a deeper desire to deconstruct the understanding of humanity as a particular community of intersubjectivity that can only be entered into by descent, for a variety of reasons, and that people jump at the opportunity to use evolution and genetics as a wedge for this deconstruction (even when their understanding of the biology involved is, in fact, poor). For one, I think some people do have this inclination to play God. They want to imagine that subjecthood could be conferred on animals, or that extraterrestrials could somehow essentially just be "humans from space." More darkly, I think people want to take humanity away from the inconvenient, especially the brain-damaged and, of course, the unborn.
The "democratic" urge which was supposed to be so humanizing actually leads to an idea that humanity is not transmitted "hierarchically" (ie, flowing from a single common font or head of the race), but rather is simply, like ethnicity, a matter of consensus; if one identifies as human and is recognized as such by other self-identified humans, then you're "in" (which implies some people could, by community consensus, have their humanity revoked.) I do think it is all, in this sense, very relevant to the abortion debate, and indeed to how that relates to the sexuality culture wars more generally. If humanity is recognized as being transmitted by descent, then the sacred nature of sexuality and its connection to procreation becomes clear enough. However, the cause of allowing this connection to be morally severed would be greatly helped if the conferring of human personhood was seen as an act or event far removed from sex. If membership is something one is "adopted" into rather than only possibly born (or, rather, conceived) into, then this helps the whole culture of death.
Finally, there are ecclesiasiological implications. Membership in the Church is analogous in most ways to membership in humanity. In entering the Church, we are entering a new community, putting on the New Man. In the Church, our Head is now Christ rather than Adam and, while not renouncing our human nature (for, indeed, Christ Himself was human and a Son of Man), we take our nature, a restored nature, a divinized nature, from Christ. But what did Christ tell us? He said that if man is not born again by water and the Holy Spirit, he cannot have eternal life. Although we are adopted as sons of daughters of God, it is really more accurate to point out that the Church is a community that also can only be entered through birth, through being born again, in baptism. Of course, the Church is not (like humanity) an intersubjectivity based on physical birth, but on spiritual birth (though, obviously, to be spiritually reborn you need to be born with a soul in the first place).
Radical individualists do not like this communal dimension to salvation, do not understand that salvation can only possibly involve re-birth into a community. But it is only entry into this new community founded on the new
intersubjectivity established by God dwelling with man, founded on
Christ incarnating in the womb of His Immaculate Mother, as New Adam and
New Eve in a new meaning-establishing encounter (that, now, of God and
Man) that can restore our nature.